Air ducts used in modern aircraft to supply ambient air to various systems situated on board the aircraft are conventionally configured as ram-air ducts having an air inlet and a diffuser. During cruising of the aircraft ambient air flows through the air inlet into the ram-air duct. In the diffuser, because of the slowing-down of the air flow, some of the dynamic pressure of the ambient air flow is converted to static pressure. As a result there arises in the ram-air duct a static pressure that is higher than the ambient pressure, the so-called ram pressure that ensures that the air flowing through the ram-air duct is supplied to an aircraft device that is to be supplied with ambient air, such as for example to a heat exchanger that is disposed in the ram-air duct. During ground operation of the aircraft, on the other hand, a mechanical feed device, such as for example a blower, ensures the desired ambient air supply through the ram-air duct to the aircraft device to be supplied with ambient air.
The air inlet of a ram-air duct may be configured in the form of a ram-air scoop or be integrated into the aircraft structure, around which flow occurs. An air inlet integrated into the aircraft structure, around which flow occurs, makes it possible to reduce the drag caused by the air inlet during cruising of the aircraft. An optimized air supply into the ram-air duct simultaneously combined with low drag may be realized for example with the aid of an NACA (National Advisory Committee for Aeronautics) inlet integrated into the aircraft surface, around which flow occurs.
With some aircraft systems that are to be supplied with ambient air there is the problem that these systems require a greater air-mass flow during ground operation of the aircraft than during cruising of the aircraft. A ram-air duct inlet that is optimized for the air-mass flow requirement of such aircraft systems during cruising would therefore have to have a relatively small cross section in order to minimize the drag caused by the air inlet during cruising of the aircraft. In contrast thereto, during ground operation of the aircraft a ram-air duct inlet cross section that is as large as possible would be advantageous to allow a blower, which during ground operation of the aircraft feeds ambient air through the ram-air duct to the aircraft system to be supplied with ambient air, to be operated at a feed pressure that is as low as possible. A blower fitted in a ram-air duct with a small ram-air duct inlet cross section that is optimized for cruising, on the other hand, has to be overdimensioned to provide the required high feed pressure, the disadvantageous result of this being that this blower has a large volume, a high weight and a high energy consumption and during operation causes a relatively large amount of noise.
The underlying object of the invention is to provide an air duct for supplying ambient air in an aircraft that enables an aircraft system, which requires a greater air-mass flow during ground operation of the aircraft than during cruising of the aircraft, to be supplied with ambient air in an energy-efficient manner and without the use of an overdimensioned feed device.